Flat oriented strand board-fiberboard composite structure and method of making the same

ABSTRACT

An oriented strand board (OSB)-fiberboard composite structure is comprised of a baseboard having three wood strand layers, the wood strands being oriented in space with respect to a board forming machine such that a core layer is comprised of wood strands oriented generally in a random or cross-machine direction and each adjacent layer is comprised of coarse and fine wood strands oriented generally in the machine direction. In a preferred embodiment, the wood strands comprising each adjacent OSB layer are formed with the coarsest strands located nearest the core layer and the finest strands are located nearest the outer surfaces of each outer board layer. The OSB-fiberboard composite product is clad with a wood fiber overlay on one major surface of the baseboard. The composite board is manufactured without warping, by providing particular OSB layer thicknesses, such that the lower OSB layer is about 25% to about 35% thicker than the OSB layer bonded to the fiberboard.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.07/670,681, filed Mar. 20, 1991, abandoned, which is acontinuation-in-part of application Ser. No. 07/503,573, filed Apr. 3,1990, now abandoned.

FIELD OF THE INVENTION

The present invention is directed to oriented strand board (OSB) and,more particularly, to an improved OSB-fiberboard composite structurehaving a fiberboard surface which will resist weathering, OSB-fiberboarddelamination and is properly balanced in multiple layer thicknesses toprevent warping, particularly cupping. The surface of the fiberboardupper layer may be readily embossed with relatively deep patterns, canmaintain sharp outside embossed corners in board or panel construction,and may be finished with paint or the like so that the product can beused as a visible siding or panelling. The fiberboard outer (top) layeris bonded to an OSB baseboard including at least three layers of OSBwherein the oriented strand board outer (bottom) layer is about 25% toabout 35% thicker than the fiberboard overlay-contacting OSB layer; anda central OSB core layer comprises about 25% to about 35% of the totalthickness of the three OSB layers, to prevent warping of the product.

BACKGROUND OF THE INVENTION

OSB is made from flakes that are created from debarked round logs byplacing the edge of a cutting knife parallel to a length of the log andthe slicing thin flakes from the log. The thickness of the flake isapproximately 0.010 to 0.030 inch. The cut flakes are subjected toforces that break the flakes into strands having a length parallel tothe grain of the wood several times the width of the strand. The strandscan be oriented on the board forming machine with the strandspredominantly oriented in a single, e.g., cross-machine direction inone, e.g., core layer and predominantly oriented in the generallyperpendicular (machine) direction in adjacent layers. The various layersare bonded together by natural or synthetic resin(s) under heat andpressure to make the finished OSB product.

The common grade of OSB is used for sheathing walls and decking roofsand floors where strength, light weight, ease of nailing, anddimensional stability under varying moisture conditions are the mostimportant attributes. In these applications, the appearance and/orweathering of the rough surfaces are not of concern since the productwill be covered with roofing, siding, or flooring. Because of theunfinished attributes of utility grade OSB, it commands a relatively lowprice in the marketplace and is sold at a discount to structural gradesof softwood plywood.

The light weight, ease of nailing, and dimensional stability of OSB areattributes much desired in siding products but, due to the irregularsurface, OSB has required surface modification before being used assiding or otherwise where aesthetics is important to the consumer. Ifthe material could be imparted with the surface smoothness, coatability,and weatherability of hardboard while retaining its other desirablestructural properties, it would be significantly improved in comparisonto the commodity structural grade. Others have pursued this objectivealong different lines with partial success.

One attempt to prevent "telegraphing" is described in Greten U.S. Pat.No. 3,098,781. The Greten '781 patent discloses a particleboard productmade from materials, such as flakes, wherein the flakes are graduated insize from the center or core to the outer surfaces, with the coarsestflakes at the core and the finer flakes, together with fines, at one orboth outer surfaces. The Greten produced particleboard is disclosed tohave the advantage of accepting an overlay of veneer, paper or plasticsheets without "telegraphing" the relatively irregular surface of theunderlying particleboard.

Similar OSB siding products are commercially sold that include aresin-bonded overlay of paper laminated to one surface. The paper canaccept a limited degree of embossing but it cannot stretch to acceptdeep embossing. When embossed beyond a certain depth, the paper rupturesfrom the tensile strain and reveals the underlying flakes. Furthermore,exposure to the weather causes irreversible swelling of the flakes inthickness which telegraphs the structure of the underlying baseboard(OSB) through the thin overlay and creates a bumpy, irregular exposedsurface. The result is an unsightly appearance of the front surface,especially of product that is unembossed or only slightly embossed.

Another example is described in Wentworth U.S. Pat. No. 4,364,984 wherewood fines are distributed on the surface of the flake baseboard (OSB)graduated with the coarsest wood fines adjacent to the flakes and thefinest on the visible surface. Since the fines are bundles of woodfibers which retain the stiffness of wood, they do not consolidate intoa tight surface, but rather, retain susceptibility to the ready entry ofwater and do not holdout paint to a satisfactory degree.

Similarly, Ufermann, et al. U.S. Pat. No. 4,068,991 discloses aparticleboard, e.g., chipboard product that includes a continuousparticle size gradient between a coarser particle core and a finerparticle surface layer wherein the particle size gradient transitionfrom one particle size to another can be continuous or step-wise.

Others have disclosed the manufacture of laminates of plywood orparticleboard with a wet-process fiberboard surface, e.g., BirminghamU.S. Pat. No. 2,343,740; Bryant 3,308,013 and Shaner, et al. U.S. Pat.No. 4,361,612 discloses forming an oriented strand board (OSB), that maybe in three or more layers, formed from a mixture of hardwood speciesand then laminating the OSB to a veneer, wet-process hardboard orplywood face panel.

One of the problems associated with the application of an overlay ontoan OSB baseboard is that of achieving a strong bond at the interfacebetween the OSB and the overlay capable of resisting weathering. Theabove-described Wentworth U.S. Pat. No. 4,364,984 suggests that a strongbond can be achieved at the interface between an OSB product and a fineparticle overlay by manufacturing the OSB with the largest OSB flakes atthe interface, and applying the overlay fine particles such that thelongest fines are disposed at the interface. Similarly, the Shaner, etal. U.S. Pat. No. 4,361,612 discloses that shorter fibers in the surfaceof an OSB product will degrade the bending strength of an OSB product.Further, the Shaner '612 patent teaches that a laminated wood productincluding a flakeboard core laminated to a wood veneer, a wet-processhardboard or a wet-process fiberboard overlay, as in typical plywoodpractice, may need a core finishing operation on a drum sander toachieve a core surface capable of good bonding to the overlay.

Bryant U.S. Pat. No. 3,308,013 suggests that a water-laid fiber sheetcontaining resin and having a basis weight of dry fiber from 30 to 750pounds per thousand square feet can be employed to mask defects inplywood, particleboard, and the like. These heavy papers have been usedto produce medium density overlain plywood that has found application inroad signs where the smooth surface accepts lettering and reflectivelaminates. High cost, limited embossability, poor weathering, and pooradhesion of coatings preclude the use of this product in sidingapplications.

It has heretofore been generally accepted by those skilled in the artthat an OSB baseboard and a fiberboard overlay will not form a good bondat their interface and that the differential in dimensional and elasticproperties of the fiberboard and OSB materials will result indelamination because of moisture cycling due to weather conditions. Thisconventional wisdom also advised against using dried board trim waste asa raw feed to the fiber pulping operation because of residual bonded andconsolidated resin. While this theory has been verified for OSBwet-process fiberboard composite structures, surprisingly andunexpectedly, excellent bonding and resistance to weathering is achievedin accordance with one embodiment of the present invention by applying afiberboard overlay by the dry process to an OSB baseboard. Additionaladvantages are achieved in the preferred embodiment by forming the OSBsuch that the smallest flakes of the OSB are disposed at the fiberboardinterface, as will be described in more detail hereinafter.

In the prior art manufacture of OSB, a warping problem was encounteredwhen the OSB was formed from three OSB layers using a screen within theplaten press for final consolidation of the three strand layers into aunitary OSB structure. It was theorized that the screen marks on the oneOSB surface layer increased the amount of effective surface area on thatOSB surface layer, thereby causing the warping problem. In order tocompensate for warpage, it was found in the prior art that warping couldbe prevented by increasing the thickness of the screen-marked (highersurface area) OSB surface layer, in comparison to the thickness of theOSB surface layer without screen marks, an amount such that thescreen-marked OSB surface layer had a thickness 15% higher than one-halfthe total thickness of the OSB surface layers surrounding the OSB corelayer. Typically, warping was prevented in prior art OSB manufacture,wherein one of the surface layers of the OSB included screenindentations, by providing a three-layer OSB product such that the top(non screenmarked) OSB layer comprised 31.6%-33.4% of the total OSBthickness; the center OSB layer comprised 42%-45% of the total OSBthickness; and the lower (screen-indented) OSB layer comprised23.4%-24.6% of the total OSB thickness. Thus, the top OSB layer (notscreen-indented) having the smaller surface area was made 15% thickerthan 1/2 the sum of top and bottom OSB layer thicknesses to preventwarping, with the central OSB layer, oriented perpendicularly to themachine direction, comprising 42%-45% of the total board thickness.

It has been found, in accordance with one embodiment of the presentinvention, that to achieve excellent embossing fidelity (the capabilityof achieving a sharp, accurate and permanent transference of anembossing plate design from an embossing plate to a board surface) in anOSB fiberboard overlay, the fiberboard overlay should be air-laid(formed by the dry process). If the fiberboard overlay applied over anOSB surface is water-laid (formed by the wet process), as suggested inthe prior art, the sharp corners and other embossing precision necessaryfor high quality transference of an embossing plate design is notpossible.

Unexpectedly, it has been found that the application of a dry layer of amixture of defibrated fiber and resin binder over an OSB surface enablesexact and precise transference of embossing plate details into thesurface of the fiberboard overlay. Further, the bonding achieved at theinterface between the OSB and the dry process fiberboard overlay, andthe resistance to weathering of the fiberboard overlay are unexpectedlybetter when the fiberboard overlay is formed into a loose, buthandleable mat formed by the dry process (the fibers are laid onto asupport surface by gravity from a mixture with air, or mechanically, andare contacted with a binder resin during the fall of fibers onto thesupport surface, and generally contain less than about 15% water) andthe fiberboard overlay and OSB layers are consolidated in a hot presssimultaneously. As set forth in more detail hereinafter, the bonding isunexpectedly higher and the boil swell values substantially lower forthe OSB-fiberboard composite products of the present invention than fora similar product that includes a fiberboard overlay applied by thetypical wet process.

Furthermore, those skilled in the art have anticipated warping of theproduct if the overlay were applied only to one surface but, inaccordance with another embodiment of the present invention, it has beenfound that the expected warping does not occur even in full size panels,e.g., 4'×8', when the fiberboard overlay is applied to only one majorsurface, and the thicknesses of the underlying OSB layers are carefullyselected, as described in more detail to follow.

The multi-layer OSB-fiberboard composite structure of the presentinvention, having one fiberboard surface layer and the other surfacelayer formed from an OSB layer without screen indentations, and havingat least two OSB layers therebetween, has substantially differentcharacteristics and physical properties from an OSB without the overlayand, therefore, was completely different in terms of possible warp orcupping during manufacture.

Initial experimental trials in the manufacture of the OSB-fiberboardcomposite of the present invention on a commercial scale, having threeOSB layers of equal thickness, and a surface layer of fiberboard overone of the outer OSB layers resulted in a board that cupped or warpedsubstantially, even with light weight overlays, e.g., 150 pounds perthousand square feet, leading to the present invention.

In accordance with another embodiment of the present invention, warpingor cupping of OSB-fiberboard composite structures can be eliminated withcareful selection of OSB layer thicknesses, regardless of whether thefiberboard layer is applied by the wet or dry process, as described inmore detail hereinafter.

SUMMARY OF THE INVENTION

The present invention combines the desirable attributes of OSB baseboardwith the embossability, ease of finishing, bonding strength, andweatherability of a fiberboard, e.g., hardboard overlay. An OSBbaseboard mat is overlain with a preformed dry fiber sheet and the twostructures are consolidated and bonded in a single hot pressing. Becauseof the unconsolidated condition of the fiber overlay before hotpressing, and, unexpectedly, due to the fiberboard overlay being formedby the dry process, deep embossing of architectural profiles arepossible without fracture of the overlay while achieving unexpectedlyprecise embossing fidelity. The dry-process fiberboard overlay mat canbe consolidated into a hardboard-like layer which has the smoothness,resistance to water penetration, weatherability, resistance to boilswell, and paint holdout of conventionally made hardboard used forsiding. In accordance with one embodiment of the present invention, whenthe OSB baseboard is manufactured such that the smallest flakes aredisposed at the OSB-fiberboard interface, the overlay masks flaketelegraphing of even smooth-surfaced, unembossed product having arelatively thin fiberboard overlay, e.g., less than about 1/8 inchthick, e.g., about 3/32 inch thick.

To achieve the full advantage of the present invention, in accordancewith one embodiment, sized board trim OSB waste can be used as feed forpulping for the dry process manufacture of the fiberboard overlay sothat defiberized fiber from the OSB baseboard trim can be refined toform the dry-process fiberboard overlay that is consolidated under heatand pressure to yield a product that has the stability, ease of working,and light weight of OSB and the architectural aesthetics, coatability,and weatherability of hardboard. The OSB-fiberboard composite structureshows no tendency to delaminate after severe moisture cycling betweenboiling water and hot oven conditions and remains free of warping over awide range of moisture environments.

Accordingly, one aspect of the present invention is to provide anoriented strand board-fiberboard composite structure that has new andunexpected resistance to delamination of the fiberboard overlay,unexpected weatherability and unexpected resistance to warping.

Another aspect of the present invention is to provide an oriented strandboard-fiberboard composite structure that includes the surfacedeformability and aesthetics of fiberboard as well as the structuralstrength of oriented strand board without separation of the fiberboardfrom the oriented strand board, wherein the fiberboard is felted by thedry process.

A further aspect of the present invention is to provide an orientedstrand board-fiberboard composite structure, wherein the oriented strandboard is formed with the smallest flakes at the fiberboard interface toprevent telegraphing of the flakes through the fiberboard surface.

Still another aspect of the present invention is to provide an orientedstrand board-fiberboard composite structure that does not warp, cup orbow upwardly at its edges upon removal from a hot press by the judiciousselection of thicknesses of the OSB layers, whether the fiberboardoverlay is formed by the wet process, e.g., water-laid, or by the dryprocess, e.g., air-laid.

The above and other aspects and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cut-away perspective view of an OSB-fiberboard compositestructure of the present invention;

FIG. 1b is a side view of the OSB-fiberboard composite structure of FIG.1a;

FIG. 2a is a profile view of the top flake layer of a conventional OSBproduct;

FIG. 2b is a profile view of the top flake layer of the preferredOSB-fiberboard composite structure of FIG. 1a;

FIG. 3a is a cut-away perspective view of a conventional OSB product inboard form utilizing strands in the top flake layer and exhibiting atelegraphed flake in the surface of a thin paper overlay;

FIG. 3b is a profile view of the conventional OSB product of FIG. 3a;

FIG. 4a is a perspective view of an OSB-fiberboard composite structureof the present invention having an embossed surface.

FIG. 4b is a profile view of the OSB-fiberboard composite structure ofFIG. 4a;

FIG. 5a is a perspective view of a molded OSB-fiberboard compositestructure of the present invention; and

FIG. 5b is a profile view of the molded OSB-fiberboard compositestructure of FIG. 5a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Low density woods, such as aspen, have been preferred for making hotpressed oriented strand boards because the higher pressure needed todevelop the board densities improves consolidation. The ratio of pressedboard density to wood density is referred to as the compaction ratio.The compaction ratio of a product may be obtained by dividing thespecific gravity of the product by the specific gravity of the wood. Forexample, a compaction ratio of about 1.23 is obtained for a producthaving a specific gravity of 0.665 which is made from Southern YellowPine having a specific gravity of 0.54. The specific gravity of thestrands usually is in the range of about 0.45 to about 0.60. For aspen,the ratio is generally in the range of 1.6 to 1.8 using oven dry weightsand green volumes.

In accordance with a preferred embodiment of the present invention, woodspecies of intermediate to high density are used to form the OSB flakesto achieve flakes that are relatively stiff and have a relatively highresistance to compression. When hot pressed, the stiff flakes in the OSBbaseboard force the overlaying dry-process fiberboard mat to undergomost of the resulting compaction, thereby developing maximum density inthe overlay. In practicing this invention, wood species having anapproximate specific gravity in the range of about 0.45 to about 0.60are preferred. Wood flakes having a specific gravity in the range ofabout 0.45 to about 0.60 generally offer compaction ratios of 1.2 to1.5. It is preferred that the final OSB-fiberboard product has anoverall specific gravity of about 0.60 to about 0.80.

The preparation of flakes is accomplished in the usual manner so as toyield strands having aspect ratios (ratio of length to width) of about 5to about 30, and moisture contents of about 15 percent or less,preferably about 1 to about 15 percent, based on the dry fiber weight.Generally, green logs having a moisture content of about 40% to about60%, green basis weight, are sliced, and dried to, for example, about 3%by weight moisture, before being screened and contacted with resin. Thestrands are screened to separate out slivers, which are particles with awidth of approximately 3/8 inch or less. To achieve the full advantageof the present invention in the forming of the OSB baseboard, theconventional practice of placing slivers in the core and larger strandson the faces is reversed. For siding, the core is made up of strandsdeposited in a random pattern or in the direction perpendicular to themachine direction, followed by larger strands adjacent to the corealigned parallel to the machine direction, followed by slivers arrangedparallel to the machine direction. The purpose of the layer of sliversis to aid in masking the large strands that lay beneath and to provide astiff layer to force the compression of the fiber overlay. Thisorientation, with the smallest strands at the fiberboard overlayinterface, achieves the best results for smooth-surfaced (non-embossed)product, particularly where the fiberboard overlay is in a layer ofabout 200 pounds of fiber, dry weight basis, per thousand square feet orless, to prevent telegraphing of flakes through the fiberboard overlay.For product containing thicker fiberboard overlays, or for product thatis embossed on the fiberboard overlay, the distribution of flakes in theOSB layer is less important.

In accordance with a preferred embodiment of the present invention,fiber for the overlay can be liberated, defiberized and refined from therough trim cut from the ends and edges of the OSB layer, from theOSB-fiberboard composite structure. Such trim is consolidated board withcured resin and wax sizing with the layered structure typical ofwaferboards and strand boards. The trim strips are chipped, e.g., in adrum chipper and steamed at pressures of about 25 to about 300 psigsaturated steam pressure for a period of about 2 minutes to about 10minutes, and refined under pressure in a pressurized refiner similar inoperation to those used for producing fiber from chips. Because of thelayered structure of OSB chips, steam readily penetrates the chip,permeates to the middle lamella (interfiber layer), softens theinterfiber layer, and permits separation of the individual fibers into apulp finer than that obtainable by refining log chips. The readypulpability of the consolidated board trim, in accordance with thepresent invention, was unexpected to those skilled in the art, and theuniform fiber that results is very compliant and readily densifies intoa hardboard layer under heat and pressure with unexpectedly preciseembossing fidelity when applied by the dry process. Furthermore, thedried trim produces fiber having a moisture content of 15 percent byweight or less, based on the dry weight of the fiber, that does notrequire drying prior to hot pressing. An elevated moisture, up to about15 percent by weight of dry fiber, contributes to the development ofconsolidation but is not a requirement for consolidation.

The strands preferably are blended with a hydrocarbon size (typicallyparaffinic or microcrystalline wax) in an amount of about 0.5% to about4.0%, preferably about 2.5% based on the dry weight of the strands; and,a binder resin, such as phenol formaldehyde resin or apolydiphenylmethyl diisocyanate (PMDI) resin, and delivered to theforming machine. The slivers are blended in a similar fashion with thesame binder and size and delivered to a forming machine. The overlayfibers are blended with wax and resin, dry-formed and laid onto asupport surface, e.g., forming belt, separately by means of forced airor mechanical means, prepressed and transferred to the top major surfaceof the OSB baseboard mat.

The OSB baseboard is formed preferably in three layers, the first andthird using air or mechanical classification to classify the particlesso that, preferably, the finest particles are the first down on theforming belt and the last down on the mat. The first layer is laid withthe strands oriented in the machine direction. The core, or central OSBlayer, is formed with randomly oriented strands or with the strandsoriented in the cross-machine direction. The third (fiberboard adjacent)layer is laid with the strands oriented in the machine direction andpreferably with graduation from coarse strands to slivers so that thesmallest strands are disposed against the fiberboard overlay. Once thethree-layered mat is formed, the preformed dry-process overlay isdeposited upon it and the total mat (OSB-fiberboard composite structure)is prepressed prior to cutting into lengths for loading into the platenpress for final consolidation.

The formulation of the furnish and the basis weight of the OSB baseboardmat and the overlay can be varied widely without going beyond the scopeof the present invention. It is preferred that a phenol formaldehyderesin or isocyante resin binder be used with microcrystalline orparaffinic waxes for sizing. The preferred furnish formulas aregenerally about 2 to about 10 percent by weight resin and about 0.5 toabout 2.5 percent by weight wax based on the dry weight of the fiber.OSB baseboard basis weight can be varied between about 900 and about2,000 pounds/thousand square feet with about 1,100 to about 1,500, eg.,1,200 pounds/thousand square feet being preferred. The dry-processfiberboard overlay basis weight can range between about 75 and about 400pounds/thousand square feet with about 200 to about 350 pounds/thousandsquare feet preferred.

Final pressing of the OSB-fiberboard prepressed composite mat to fullyconsolidate the composite board preferably should be limited to preventover compaction of the board which increases thickness swellingpotential. Although the pressed board will typically be between about0.25 and about 1.0 inch thick, the preferred product is about 0.400 toabout 0.500 inch thick with an overall density in the range of about 38to about 47 pounds per cubic foot (specific gravity in the range ofabout 0.60 to about 0.75 oven dry weight and air dry volume basis). Thisleads to a compaction ratio of approximately 1.3 for a species such assouthern yellow pine. Under these conditions, the dry-process fiberboardoverlay will average about 50 to about 55 pounds per cubic foot, whichis typical for hardboard siding. At a given overall product density, thedensity of the hardboard overlay can be increased by using fibermoisture contents of approximately 15 percent or less, e.g., 12 percentby weight of dry fiber, and flake moisture contents of about 8 percentor less, e.g., 6 percent by weight of dry flakes. This leads tocompliant fiber and relatively stiff flakes which foster tighter overlaysurfaces having properties of excellent embossing fidelity, bonding tothe OSB baseboard, and weatherability.

In those instances of embossing deep enough to cause overdensificationof the baseboard along deepest embossing contours, the hot press can beoutfitted with a backer plate that is roughly contoured to complementthe contours of the top embossing plate. In this manner, the top andbottom embossing plates become a die set which molds the OSB baseboardto a shape that permits deep embossing of the top fiberboard overlaysurface while creating sharp outside corners in the overlay fibers andnear uniform density in the baseboard, with a contoured OSB baseboard,instead of overdensification of fiberboard along lines of deepestembossing. The molded (contoured) profile of the OSB baseboard can besanded on the back surface, if necessary, to restore a flat surface thatfacilitates installation against a flat support surface, e.g., assiding.

A further benefit of molding the product in a die set to contour bothmajor outer surfaces of the OSB-fiberboard composite structure is theopportunity to densify specific regions of the product that willsubsequently be cut or shaped by cutters to facilitate properinstallation. Densification improves machinability and the quality ofresulting cut surfaces and also enhances the resistance of any cutsurfaces to the entry of water.

Turning now to the drawings, and initially to FIG. 1a, a portion of anOSB-fiberboard composite structure 100 is cut away to show severaldetails of its construction. Bottom flake layer 101 is comprised of woodstrands and slivers oriented generally in the machine direction, with astrand fraction 103 on its uppermost surface, nearest middle flake orcore layer 105. Middle flake or core layer 105 is comprised of woodstrands oriented generally in the cross-machine direction. Top flakelayer 107 is disposed above middle flake layer 105 and is comprised ofwood strands and slivers, oriented generally in the machine directionpreferably with a strand fraction nearest the middle flake layer 105 anda sliver fraction 109 disposed on the uppermost surface of flake layer105. Construction of the OSB-fiberboard composite structure 100 iscompleted with the addition of a dry-process fiberboard overlay 111 uponthe top surface of the top flake layer 107.

As shown in FIG. 1b, the OSB-fiberboard composite structure 100 iscomprised of bottom flake layer 101, middle flake or core layer 105, topflake layer 107 and dry-process fiberboard overlay layer 111, whereinsliver fractions 109 and 109a are seen at the upper surface of top flakelayer 107 and the lower portion of the bottom flake layer 101,respectively. Strand fractions 103 and 103a are seen at the lowerportion of top flake layer 107 and the upper portion of bottom flakelayer 101, respectively.

FIG. 2a depicts a profile of a conventional top flake layer 200 havingits strand fraction 201 oriented near the top surface and its sliverfraction 203 oriented near its bottom surface.

FIG. 2b depicts a profile of top flake layer 107 from the board shown inFIG. 1a, having a sliver fraction 109 oriented near the fiberboardinterface and a strand fraction 103a oriented near the bottom portion ofthe flake layer 107.

FIGS. 3a and 3b show a conventional OSB product 300 in cut-awayperspective and profile, respectively. The conventional product 300 iscomprised of bottom flake layer 301, middle flake or core layer 303, topflake layer 305 and a thin paper overlay 313. Wood strands 307 and 311are oriented generally in the machine direction while strands 309 areoriented generally in the cross-machine direction. Telegraphed flake 315is an unsightly blemish in the thin paper overlay 313 and is one of thedisadvantages seen in the use of conventional OSB product 300 inapplications where appearance is important.

FIGS. 4a and 4b depict in perspective and profile view, respectively, anembossed OSB-fiberboard composite structure 400 of the presentinvention. The OSB-fiberboard composite structure 400 includes a bottomflake layer 401, a middle flake or core layer 403, a top flake layer 405and a dry-process fiberboard overlay layer 407 capable of receiving asuitable embossing impression, thereby exhibiting embossed surface 409.

FIGS. 5a and 5b, respectively, depict a perspective and profile view ofa molded dry process lain OSB-fiberboard composite structure, having adry-process fiberboard overlay surface over a three layer flakeconstruction OSB.

The examples outlined below describe the manufacture of theOSB-fiberboard composite structure of one embodiment of the presentinvention using batch equipment, but non-embossed product also may bemade using continuous equipment and continuous presses. In a continuousprocess, the surface layers are not separately screened to provide fineand coarse fractions but the distribution of the strands with standarddistribution equipment will cause a transition area between the widerchips and the slivers. The examples are not intended to limit the scopeof the invention.

EXAMPLE NO. 1

In this example, green southern yellow pine roundwood bolts were flakedin a pilot plant disk flaker to a thickness of 0.020 inch. The resultingstrands had a length less than about 3 inches and a width less thanabout 1 inch, with the average being about 1/2 inch in width. Thesestrands were dried in an oven to 3 percent moisture content and screenedinto two fractions, one with a width of over 3/8 inch (strands) and onewith a width less than 3/8 inch (slivers). These two fractions werehandled separately thereafter. A screen analysis of the sliver fractionusing a Ro-Tap analyzer yielded the weight fractions shown below:

    ______________________________________                                        Screen Opening Percent Retained                                               ______________________________________                                        0.371"         0.0                                                            0.185"         7.2                                                            0.131"         27.3                                                           0.093"         19.4                                                           0.046"         31.2                                                           < 0.046"       14.9                                                           Total          100.0                                                          ______________________________________                                    

The wider fraction of strands was blended with about 6% phenolformaldehyde resin and about 2% paraffinic wax applied as an aqueousemulsion.

The sliver fraction, which comprised about 1/3 of the surface flakefurnish, was blended with about 6% phenol formaldehyde resole resinformulated for OSB bonding and with about 2% emulsified paraffinic wax,both based on the dry wood weight. The use of phenolic resin in thesliver fraction prevents contact between the back of the board and thepress platen which could lead to sticking in the press if isocyanateresin were used.

Fiber for the dry-felted fiber mat was produced from OSB board trimwaste that had been chipped by a commercial drum chipper, steamed insaturated steam for about 5 minutes at 125 psig, and refined in acommercial single disk pressurized refiner coupled to a digester. Thefiber exited the refiner at 12% moisture content and 2.5% by weight, dryfiber basis, molten paraffinic wax was added. The fiber then was driedto about 5% moisture content in order to avoid blistering when deeplyembossed. Once dry, the fiber was blended with about 4% neat PMDI.

The OSB was produced from the foregoing materials, first by laying downslivers having a basis weight of about 130 to about 170 pounds/thousandsquare feet by dropping them onto an orienting device comprised of metalstrips on edge and arranged in parallel to form a series of slotsthrough which the slivers would fall. This oriented the slivers in adirection generally parallel to the direction of the slots. The firstlayer of slivers was oriented in the machine direction. On top of thesliver layer was deposited a layer of larger strands oriented in themachine direction. This second, strand layer had a basis weight of about275 to about 355 pounds/thousand square feet. A core layer was depositednext by changing the orientation to the cross-machine direction. Thecore layer had a basis weight of about 350 to about 430 pounds/thousandsquare feet. On top of the core layer was deposited a layer of widestrands oriented in the machine direction. This fourth layer had a basisweight of about 275 to about 335 pounds/thousand square feet. The fifthlayer was deposited as slivers oriented in the machine direction. Thisfifth, sliver layer had a basis weight of about 130 to about 170pounds/thousand square feet.

The dry-felted fiber overlay mat was formed by dropping fiber through acoarse screen onto a fine screen and thereafter prepressing the mat toreduce its thickness about in half. The basis weight of the dry-processfiber mat was 100 pounds/thousand square feet. The dry-formed mat wastransferred to the top surface of the OSB baseboard mat and loaded intoa hot press for final consolidation to provide a composite board havingan overall basis weight of 1,500 pounds/thousand square feet.

The press cycle used a hydraulic press with heated platens at 750 psigpressure on the mat and at 417° F. for 5 minutes in order to consolidateall layers of the composite board. The 5 minutes press cycle durationincluded a decompression cycle of 20 seconds to permit releasing theboard from the press without delamination or blistering. The density ofthe product was 41.5 pounds per cubic foot at an overall thickness of0.440 inch. The press plate was smooth and treated with a release agentfor PMDI before pressing.

EXAMPLE NO. 2

A board was made according to the procedures in Example No. 1 exceptthat the basis weight of the dry-felted fiber mat was 150pounds/thousand square feet. The basis weight of the OSB core for thisExample, as well as Examples 3-6, was decreased in an amount sufficientto provide a consistent overall basis weight of 1,500 pounds/thousandsquare feet.

EXAMPLE NO. 3

A board was made according to the procedures in Example No. 1 exceptthat the basis weight of the dry-felted fiber mat was 200pounds/thousand square feet.

EXAMPLE NO. 4

A board was made according to the procedures in Example No. 1 exceptthat the basis weight of the dry-felted fiber mat was 250pounds/thousand square feet.

EXAMPLE NO. 5

A board was made according to the procedures in Example No. 1 exceptthat the basis weight of the dry-felted fiber mat was 300pounds/thousand square feet.

EXAMPLE NO. 6

A board was made according to the procedures in Example No. 1 exceptthat the basis weight of the dry-felted fiber mat was 350pounds/thousand square feet.

The six boards described above were coated with a conventional hardboardthermosetting acrylic primer and entered into an accelerated agingchamber specifically designed to cause swelling of wood composite sidingproducts that are vulnerable to swelling. In this chamber, verticallyoriented specimens are subjected to 12 hours of water spray on the frontface followed by heat of 135° F. for 12 hours. The chamber remains humidduring the early stages of the dry cycle which increases the swellingcapacity of water that has entered the specimen. The cycling procedurecan be adjusted to repeat the wet and dry cycles for as many periods asmay be required to test and compare the composite oriented strand boardproducts. After 19 cycles, telegraphing was noted on the board havingoverlay basis weights of 100 and 150 pounds/thousand square feet.Telegraphing was minimal on boards having dry-felted fiberboard overlayof 200 pounds/thousand square feet and no telegraphing was noted onboards having a dry-felted fiberboard overlay with basis weights greaterthan 200 pounds/thousand square feet. In commercial size plant trails,cupping (the composite board turning upwardly at the longitudinal edgeswith the exposed OSB layer convex) or bowing (the composite boardturning upwardly at the transverse edges with the exposed OSB surfaceconvex) was noted in all boards, with the cupping or bowing moreprominent in the boards having higher basis weight overlays.

It is noted that smooth, unembossed OSB made according to the inventionwith dry-felted fiber overlays having basis weights of about 200 toabout 300 pounds/thousand square feet will weather free of telegraphing.The combination of low compaction ratio (about 1.3) and thick overlayprevents the excessive thickness swelling of the baseboard flakes.Embossed composite OSB products of the present invention can have awider ranging basis weight for the dry-felted fiber overlay, e.g., about100 to about 300 pounds per thousand square feet, and weather free oftelegraphing and warping.

Initial laboratory trials indicated that warping would not be a problem,despite the unbalanced construction caused by applying the fiberboardoverlay to the front surface without a corresponding overlay applied tothe back surface, so long as the basis weight of the dry-felted overlayis limited to about 300 pounds/thousand square feet or less.Particularly, when the overlay exceeds about 300 pounds/thousand squarefeet, fiberboard overlay hygroexpansion forces are sufficient to preventsevere warping of the board.

EXAMPLE NO. 7

Experiments were conducted to compare wet- and dry-formed fiberboardoverlay layers for identical oriented strand boards. It was found thatdry-formed overlay mats are unexpectedly better than wet-formed overlaymats in terms of surface quality, including embossing fidelity andpaintability, and bonding properties.

The following conditions were used in the preparation of theOSB-fiberboard composite structures:

    ______________________________________                                        Moisture content of                                                                             6%                                                          dry-formed overlay fiber                                                      Moisture content of                                                                             6-10%                                                       wet-formed overlay fiber                                                      Resin content of  5% PMDI                                                     dry-formed overlay fiber                                                      Resin content of  5% Phenol Formaldehyde                                      wet-formed overlay fiber                                                      Moisture content of                                                                             6.5%-7.5%                                                   flakes after blended                                                          with resins                                                                   Resin content of  6% Borden LH96B Resin                                       surface flakes (fine                                                          and large flakes)                                                             Resin content of  4% Mobay PMDI                                               core flakes                                                                   Wax content of    1.5% paraffin wax                                           all flakes                                                                    Pre-press sealer  emulsion                                                                      2g solids/ft2                                                                 R & H E-2761                                                Pressing temperature                                                                            420° F.                                              Pressing time     5 min.                                                      ______________________________________                                    

The same fiber prepared from OSB trim was used for both formingprocesses. In the wet-forming process, the fiber was mixed with tapwater, and the phenol formaldehyde (PF) resin was precipitated into theslurry with acetic acid. The mats were oven dried at 250° F. for 45minutes and left at room temperature for 24 hours. Thicknesses ofwet-formed mats after drying and dry-formed mats were 3/4" and 1/2",respectively. A "Triple Four Pine Textured" die set was used for themoldability study. All boards were pressed with a 20"×20" laboratorypress. For each forming process, three boards were made.

The surface quality was evaluated by visual examination. Thepaintability was evaluated by coating specimens with a Rohm & Haasprimer. The bonding properties were tested by boiling 2"×2" specimensfor one hour followed by oven-drying at 225° F. for 12 hours.

While fiber mats formed by both processes can be bound to the OSBsubstrate, the dry-formed overlay mat was unexpectedly superior to thewet-formed mat in surface quality. The tightness of the fiberboardoverlay surfaces from the dry-forming process is much greater than thatof wet-formed fiberboard overlay surfaces. Therefore, the dry-formedoverlay shows a smoother surface, while the wet-formed overlay presentsa rougher surface. The difference is more distinct in areas adjacent todeeply grooved or curved areas.

Paintability:

OSB specimens overlaid with dry-formed fiberboard mats exhibit betterpaint hold-out compared to those overlaid with wet-formed mats. It ismore distinct in areas adjacent to the guide line groovings, where thespecific gravity is considerably lower.

Bonding Properties:

Skin layers, which could be easily peeled off, were found on surfaces ofwet-formed fiberboard overlay mats. Also, the wet-formed fiberboardoverlays could be separated from OSB substrates. These are indicationsof resin pre-cure. The high amount of heat energy used to dry wet-formedmats could cause curing of the resin in fiberboard surface layers ofmats.

Table 1 shows differences in caliper swelling after one hour boilingbetween the two mat forming processes. The wet-formed OSB-fiberboardcomposite structure swelled a full 0.1 inch more in the fiberboardoverlay than the dry-process fiberboard overlay. The average caliperswelling of OSB overlaid with dry-formed mats is significantly andunexpectedly lower than that of OSB overlaid with wet-formed mats. Afterboiling, complete separation of wet-formed overlays from substrates wasfound, whereas, no delamination occurred in the OSB dry-processfiberboard composite structure.

                  TABLE I                                                         ______________________________________                                        Caliper Swelling Of Dry-Formed And Wet-Formed                                 Overlay Mats After One Hour Boiling                                                      Average Caliper Swelling (%)                                       ______________________________________                                        Wet-Formed   49.667                                                           Dry-Formed   31.556                                                           ______________________________________                                    

Embossing Fidelity:

OSB specimens overlaid with dry-formed fiberboard mats exhibitedvisually distinctly better embossing fidelity than OSB/wet-processfiberboard structures. Sharp, precise transference of the details of theembossing plate, with transference of sharp corners was achieved withthe dry-process fiberboard overlays but not with wet-process fiberboardoverlays.

Bonding Strength:

The OSB specimens overlaid with dry-formed fiberboard mats had aninternal bonding strength of 90 psig vs. 78 psig for OSB specimensoverlaid with the wet-process mats.

One of the problems encountered by Applicants in applying a singlefiberboard overlay over one major surface of a balanced three layeroriented strand board was that upon exiting the hot consolidation press,the product cupped, or turned up at its longitudinal edges. Incommercial trials, this cupping occurred even at 150 pounds per thousandsquare feet fiberboard overlay basis weights. For example, a compositeboard having a top fiberboard layer with a basis weight of 300 poundsper thousand square feet (Lb/MSF) over a three layer OSB:428 Lb/MSF-367Lb/MSF-428 Lb/MSF, cupped severely at its longitudinal sides uponremoval from the heated consolidation press after all layers wereconsolidated simultaneously in the hot press. Attempts to approximatelybalance the composite board by providing substantially less strandthickness in the OSB layer directly under the fiberboard overlay, e.g.,100 Lb/MSF, so that closer to the same weight of material is above andbelow the 367 Lb/MSF core, will not provide a composite board free fromwarp or cupping. Surprisingly, it was found for a 300 Lb/MSF fiberboardoverlay, best results were achieved when the OSB layers forming the OSBbaseboard have thicknesses such that 30% of the OSB strands are providedin the layer interfacing the fiberboard overlay; 31% of the OSB strandsare provided in the OSB core layer; and 39% of the OSB strands areprovided in the exposed OSB (back) layer.

Similar, non-warping results can be achieved with either wet process ordry-felted fiberboard mat overlays, so long as the center OSB (core)layer 105, having strands in a random or cross-machine direction,comprises about 25% to about 35% of the total thickness of the three OSBlayers combined, and the exposed (back) OSB layer 101 is about 25% toabout 35% thicker than the OSB layer 107 bonded to the fiberboardoverlay 111 (after consolidation). These non-warp results apply, forthese thickness relationships over the full range of 75-400 Lb/MSFfiberboard overlays. Generally, the fiberboard overlay will have athickness about 50% to about 125% the thickness of the OSB core layer. Aminimum core thickness of about 1/16 inch provides sufficient structuralintegrity to the composite OSB structure of the present invention forits use as a siding or panelling product. In a preferred embodiment, thebaseboard comprises three layers--the OSB layer adjacent to thefiberboard overlay comprising 27.7% to about 33.3% of the totalbaseboard thickness; the OSB core layer con, rising about 25% to about35% of the total baseboard thickness; and the bottom OSB layercomprising about 36.1% to about 43.1% of the total baseboard thickness.

Examples of useful thicknesses to provide non-warping composite-OSBproducts, wherein the fiberboard mat overlay can be formed by awet-laying operation, or by dry-felting are as follows:

EXAMPLE NO. 8

    ______________________________________                                        Example No. 8                                                                 Layer             Thickness                                                   ______________________________________                                        Wet-laid fiberboard                                                                             0.070 inch                                                  overlay layer 111                                                             Machine direction 0.092 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.070 inch                                                  direction core 105                                                                              (25% of OSB layers)                                         Machine direction exposed                                                                       0.120 inch                                                  back OSB layer 101                                                                              (30% thicker than layer 107)                                OSB Total         0.282                                                       ______________________________________                                    

EXAMPLE NO. 9

    ______________________________________                                        Example No. 9                                                                 Layer             Thickness                                                   ______________________________________                                        Dry-felted fiberboard                                                                           0.110 inch                                                  overlay layer 111                                                             Machine direction 0.089 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.080 inch                                                  direction core 105                                                                              (28% of OSB layers)                                         Machine direction exposed                                                                       0.117 inch                                                  back OSB layer 101                                                                              (30% thicker than layer 107)                                OSB Total         0.286                                                       ______________________________________                                    

EXAMPLE NO. 10

    ______________________________________                                        Example No. 10                                                                Layer             Thickness                                                   ______________________________________                                        Wet-laid fiberboard                                                                             0.080 inch                                                  overlay layer 111                                                             Machine direction 0.071 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.090 inch                                                  direction core 105                                                                              (35% of OSB layers)                                         Machine direction exposed                                                                       0.096 inch                                                  back OSB layer 101                                                                              (35% thicker than layer 107)                                OSB Total         0.257                                                       ______________________________________                                    

EXAMPLE NO. 11

    ______________________________________                                        Example No. 11                                                                Layer             Thickness                                                   ______________________________________                                        Dry-felted fiberboard                                                                           0.090 inch                                                  overlay layer 111                                                             Machine direction 0.097 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.100 inch                                                  direction core 105                                                                              (31% of OSB layers)                                         Machine direction exposed                                                                       0.126 inch                                                  back OSB layer 101                                                                              (30% thicker than layer 107)                                OSB Total         0.323                                                       ______________________________________                                    

EXAMPLE NO. 12

    ______________________________________                                        Example No. 12                                                                Layer             Thickness                                                   ______________________________________                                        Wet-laid fiberboard                                                                             0.100 inch                                                  overlay layer 111                                                             Machine direction 0.095 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.120 inch                                                  direction core 105                                                                              (35% of OSB layers)                                         Machine direction exposed                                                                       0.128 inch                                                  back OSB layer 101                                                                              (35% thicker than layer 107)                                OSB Total         0.343                                                       ______________________________________                                    

EXAMPLE NO. 13

    ______________________________________                                        Example No. 13                                                                Layer             Thickness                                                   ______________________________________                                        Dry-felted fiberboard                                                                           0.085 inch                                                  overlay layer 111                                                             Machine direction 0.137 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.130 inch                                                  direction core 105                                                                              (29% of OSB layers)                                         Machine direction exposed                                                                       0.182 inch                                                  back OSB layer 101                                                                              (33% thicker than layer 107)                                OSB Total         0.449                                                       ______________________________________                                    

EXAMPLE NO. 14

    ______________________________________                                        Example No. 14                                                                Layer             Thickness                                                   ______________________________________                                        Dry-felted fiberboard                                                                           0.095 inch                                                  overlay layer 111                                                             Machine direction 0.121 inch                                                  OSB layer 107                                                                 Random or cross-machine                                                                         0.135 inch                                                  direction core 105                                                                              (33% of OSB layers)                                         Machine direction exposed                                                                       0.153 inch                                                  back OSB layer 101                                                                              (27% thicker than layer 107)                                OSB Total         0.409                                                       ______________________________________                                    

It should be understood that the present disclosure has been made onlyby way of the preferred embodiments and that numerous changes in detailsof construction, combination and arrangement of parts can be resorted towithout departing from the spirit and scope of the invention ashereunder claimed.

What is claimed is:
 1. A non-warping oriented strand board-fiberboardcomposite structure comprising a multi-layer oriented strand baseboardhaving a plurality of wood strand layers, including a central layerformed from wood strands oriented generally in a random or cross-machinedirection sandwiched between two adjacent layers formed from woodstrands oriented in a direction generally perpendicular to said centrallayer; and a wood fiber overlay layer bonded to one of the strand boardlayers, wherein the thickness of the central strand board layercomprises about 25% to about 35% of the total thickness of the centraland adjacent strand board layers and the strand board layer adjacent tothe central layer that is not in contact with the fiberboard overlay isabout 25% to about 35% thicker than the strand board layer in contactwith the fiberboard overlay.
 2. The board of claim 1, wherein said woodstrands are comprised of flakes and slivers having a specific gravity ofabout 0.45 to about 0.60 having a moisture content of about 1% to about15% by weight, based on the dry fiber weight.
 3. The board of claim 1,wherein the wood fiber overlay layer is dry-felted and preformedseparately from the baseboard and thereafter transferred onto thebaseboard as a mat prior to pressing the baseboard and the matsimultaneously under heat and pressure.
 4. The board of claim 1, whereinthe wood fiber overlay layer is wet-formed and preformed separately fromthe baseboard and thereafter transferred onto the baseboard as a matprior to pressing the baseboard and the mat simultaneously under heatand pressure.
 5. The board of claim 1, wherein the wood strands arebonded with a resin selected from the group consisting of phenolformaldehyde resin and polydiphenylmethyl diisocyanate resin and whereinthe resin is added to the strands in an amount of about 2% to about 10%by weight of the dry weight of strands.
 6. The board of claim 3, whereinthe dry-felted wood fiber overlay layer is bonded with a resin selectedfrom the group consisting of phenol formaldehyde resin,polydiphenylmethyl diisocyanate resin and mixtures thereof in an amountof about 2% to about 10% by weight, based on the dry fiber weight. 7.The board of claim 1, wherein the strands are sized with a hydrocarbonwax selected from the group consisting of paraffinic wax,microcrystalline wax and mixtures thereof in an amount of about 0.5% toabout 2% by weight of the dry weight of the wood strands.
 8. The boardof claim 1, wherein the overlay wood fibers are sized with a hydrocarbonwax selected from the group consisting of paraffinic wax,microcrystalline wax and mixtures thereof in an amount of about 0.5% toabout 4% by weight of the dry weight of fibers.
 9. The board of claim 1,wherein the basis weight of the oriented strand baseboard is in therange of about 1,000 pounds/thousand square feet to about 2,000pounds/thousand square feet.
 10. The board of claim 3, wherein the basisweight of the dry-felted fiberboard overlay is in the range of about 75pounds/thousand square feet to about 400 pounds/thousand square feet.11. The board of claim 1, wherein the composite structure is hot pressedbetween two heated dies thereby conforming both the oriented strandboard layers and the fiberboard layer to a predetermined shape includingdeep embossing of the fiberboard layer including sharp outside cornersin the fiberboard overlay layer, while contouring the oriented strandbaseboard layers and maintaining near uniform density in the baseboard.12. The board of claim 1, wherein the composite structure is hot pressedbetween two heated dies to selectively densify specific regions of theboard thereby enhancing machinability of the product in said specificregions and increasing resistance to water entry into the densifiedregions.
 13. The board of claim 1, wherein all layers of the compositestructure have been consolidated under heat and pressure in a singlepress cycle.
 14. A non-warping oriented strand board-fiberboardcomposite structure comprising a plurality of layers of a man-madeboard, each layer loosely formed separately and disposed one above theother, wherein a top layer is a fiberboard layer bonded to a first layerof oriented strand board having wood strands of varying dimensionsoriented in the machine direction; said first strand board layer bondedto a core layer of oriented strand board having wood strands oriented inthe machine direction; said core layer being bonded to a bottom orientedstrand board layer having strands oriented in the machine direction,wherein all layers are pressed simultaneously under heat and pressure toform a composite structure wherein the bottom strand board layer isabout 25% to about 35% thicker than the first strand board layer, andwherein the core layer of strand board is about 25% to about 35% of thethickness of all strand board layers.
 15. The board of claim 14, whereinsaid wood strands are comprised of flakes and slivers having a specificgravity of about 0.45 to about 0.60 and having a moisture content ofabout 1% to about 15% by weight of the dry fibers.
 16. The board ofclaim 14, wherein the top fiberboard layer is preformed separately fromthe strand board layers such that the wood fibers of the top layer areof sufficient length to provide interlocking of fiberboard fibers forhandleability so that the preformed fiberboard can be positioned ontothe first oriented strand board layer prior to pressing all layerssimultaneously under heat and pressure.
 17. The board of claim 14,wherein the wood strands of each oriented strand board layer are bondedwith a resin selected from the group consisting of phenol formaldehyderesin and polydiphenylmethyl diisocyanate resin and which is added tothe strands in an amount of about 2 to about 10 weight percent of thedry weight of strands.
 18. The board of claim 14, wherein the woodstrands are formed with the coarsest strands farthest from thefiberboard layer.
 19. The board of claim 14, wherein the thickness ofeach strand board layer is related as follows:first strand boardlayer=27.7-33.3% of total thickness of strand board layers core strandboard layer=25-35% of total thickness of strand board layers bottomstrand board layer=36.1-43.1% of total thickness of strand boardlayers,and wherein the bottom strand board layer is about 25% to about35% thicker than the first strand board layer.
 20. The board of claim19, wherein the top fiberboard layer has a thickness about 50% to aboutthe thickness of the core layer.